Silicon photonics has the potential to change many aspects of data transport, but has not yet substantially impacted long haul transport. Part of the reason for this is that modulators in this platform are “chirped” in the sense that they generate signals that exhibit chirp. As is well known, chirp can degrade spectral efficiency and long haul transport characteristics.
In silicon and indium phosphide (InP) photonic integrated circuits, electro-optic modulators can create optical phase shifting by using the Kramers-Kronig effect, where a change in optical loss induces a change in optical path length within an optical waveguide. This approach is effective in creating an electro-optic response, often referred to as the plasma-dispersion effect, in materials that do not have an intrinsic electro-optic response. When used for optical modulation within a push-pull Mach-Zehnder modulator (MZM), however, there is a resulting change in modulator optical path length during signal modulation that results in a chirped transmission signal. As mentioned, modulator chirp can be detrimental to system spectral efficiency, system scalability, and transmission performance.